1. Field of the Invention
The present invention relates to semiconductor device packaging. More particularly, the present invention relates to a method for attaching a singulated good die, determined to be a good die by the probe testing thereof while in wafer form, not to be confused with known-good-die (KGD), which is a die known to be good after the burn-in and testing thereof, to a leadframe by applying a liquid or paste mixture of unreacted monomers or polymers or a diluted resin deposited in a predetermined pattern on the active surface of the semiconductor device, the leadframe, or both, after the die has been separated from the wafer and probe tested.
2. State of the Art
Well known types of semiconductor devices are connected to components, typically a leadframe, and subsequently encapsulated in thermo-setting plastic materials for use in a wide variety of applications. A conventional type leadframe is typically formed from a single, continuous sheet of metal, typically using metal stamping operations. The leadframe includes an outer supporting frame, a central semiconductor device supporting pad (die paddle) and a plurality of lead fingers, each lead finger having, in turn, a terminal bonding portion near the die paddle on which the semiconductor device is located. Ultimately, the outer supporting frame portions of the leadframe are removed after wire bonds are connected between the contact pads of the semiconductor device and the lead fingers of the leadframe and the encapsulation of the semiconductor device and portions of the lead fingers.
In the assembly of semiconductor devices utilizing such leadframes, a semiconductor device is secured to the die paddle (such as by a solder or epoxy die-attach material, although a double-sided adhesive tape has also been suggested in the art) and then the entire leadframe, with the semiconductor device thereon, is placed into a wire bonding apparatus, including a clamp assembly for holding the leadframe and semiconductor device assembly, and clamping the lead fingers for bonding.
In a standard wire bonding process, the bond wires are attached, one at a time, from each bond pad on the semiconductor device to a lead finger. The bond wires are generally attached through one of three industry-standard wire bonding techniques: ultrasonic bondingxe2x80x94using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bondingxe2x80x94using a combination of pressure and elevated temperature to form a weld; and thermosonic bondingxe2x80x94using a combination of pressure, elevated temperature, and ultrasonic vibration bursts.
U.S. Pat. No. 4,862,245, issued Aug. 29, 1989, to Pashby et al., illustrates a so-called xe2x80x9cleads over chipxe2x80x9d arrangement (xe2x80x9cLOCxe2x80x9d) on the semiconductor die. A plurality of lead fingers extend over the active surface of a semiconductor device toward a line of bond pads wherein bond wires make the electrical connection between the lead fingers and the bond pads. An alpha barrier, such as a polyimide (for example, Kapton(trademark)) tape, is adhered between the semiconductor die and the lead fingers. This configuration, which eliminates the use of the previously-referenced central die attach pad (die paddle), may assist in limiting the ingress of corrosive environment contaminants, achieve a larger portion of the lead finger path length encapsulated in the packaging material, and reduce electrical resistance caused by the bond wires (i.e. the longer the bond wire, the higher the resistance) and potential wire sweep problems aggravated by long wire loops. Clearly, such is in contrast to a conventional leadframe and semiconductor die wherein a larger semiconductor die cannot be encapsulated to form a smaller package assembly, as the overall size of the conventional leadframe having lead fingers extending adjacent the semiconductor device governs the size of the encapsulated package.
The typical method for attaching the die to the lead fingers in a LOC configuration is to use an adhesive tape having a thermoplastic adhesive on both sides. The face of the die and the lead fingers are attached to the adhesive tape using heat and pressure. This process is expensive and complicated. The tape must be precisely attached to the lead fingers and die face so that the bond pads on the die face are not covered by the adhesive tape. In addition, different sizes of tapes and different tape punches must be used for each die size.
U.S. Pat. No. 5,286,679, issued Feb. 15, 1994, to Farnworth et al., and assigned to the assignee of the present invention illustrates a method for attaching a die to a leadframe comprising the steps of: forming an adhesive layer on a semiconductor wafer; patterning the adhesive layer to clear streets for saw cutting and to clear wire bond pads; separating the dice from the wafer; heating the adhesive layer; and attaching the lead fingers by pressing the lead fingers and die together. In an alternative embodiment, the adhesive layer is applied to the lead fingers of the leadframe rather than to the wafer. The adhesive used is either a thermoplastic adhesive, such as a polyimide, or a thermoset adhesive, such as a phenolic resin. Suggested methods for depositing and patterning the adhesive are: hot screen printing, cold screen printing, resist etch back, and photopatterning. One problem with such a method is that the adhesive layer is applied and patterned to all dice on a wafer prior to sawing. It is not unusual for the yield rate in die production to be low. Therefore, adhesive is applied to many dice that are already known to be defective after the probe testing thereof or that will be subsequently rejected after singulation due to a failure to meet acceptable performance criteria. This results in material waste and decreased efficiency. Alternately, the ""679 patent discloses the application and patterning of adhesive after the dice have been singulated. However, such requires the adhesive to be applied and subsequently patterned, rather than adhesive applied in a predetermined pattern to the singulated die in a single step.
In addition, it is difficult to rework or fix a die after the lead fingers have been attached to the die face if a thermoplastic or thermoset adhesive has been used. If a thermoset adhesive was used, the lead fingers are permanently attached to the die face. If a thermoplastic adhesive was used, it is necessary to reheat the die and leadframe at a substantially elevated temperature. Alternately, the adhesive has been applied to the lead fingers of the leadframes which have been subsequently bonded to the active surface of the semiconductor device before testing of the semiconductor device has been completed, thereby making it difficult or impossible to rework the semiconductor device if found to be defective during testing.
Thus, it would be advantageous to develop a process for packaging semiconductor devices that would reduce process steps and reduce material waste.
The present invention relates to a method for attaching a singulated semiconductor die or batches of singulated semiconductor dice to a leadframe by applying a liquid or paste mixture of unreacted monomers or polymers or a diluted resin deposited in a predetermined pattern on the die face, the leadframe, or both, after the semiconductor die has been separated from the wafer and tested sufficiently before or after separation to be referred to as a good die.
The method of the present invention comprises the steps of separating individual dice from a wafer; testing the dice or, optionally, separating dice probe tested and found to be acceptable; applying adhesive in a predetermined pattern to the good die or expected to be good dice from the testing thereof; and attaching the leadframe to the dice, either a die singly or in batches of singulated dice. An intermediate curing step may be used prior to attaching the leadframe to the dice. A post cure step may also be used after the leadframe has been attached to the dice. The adhesive is applied in a predetermined pattern to the die face or the lead fingers of the leadframe or both. In addition, the adhesive is applied as a wet film. Therefore, it is possible to remove the leadframe from the die for reworking if the die is subsequently determined to be bad. Because the adhesive is applied only to good dice determined from the testing thereof, time and material are saved as opposed to a process that applies adhesive to all dice in a wafer or to singulated dice wherein the adhesive is first applied to the dice and patterned thereafter.